1. Field of the Invention
The present invention relates to improvements in a method of making a poly-V pulley from a metal plate, and more particularly to an improvement in a compression process in a poly-V pulley making method capable of forming a grooved wall having even or regular pitch between the groove valleys without any distortion.
2. Description of the Prior Art
Recently, various kinds of poly-V pulleys have been developed to be used for poly-V belts instead of V-pulleys conventionally well known. The pulleys have in their outer surfaces a plurality of narrow V-grooves along the belt travelling direction.
U.S. Pat. No. 3,977,264 (Nolte V. Sproul) discloses a technique of making a poly-V pulley made of a metal plate. However, this disclosure teaches only a basic principle of making poly-V pulleys; pressing, compression and finishing processes are applied to a cylindrical flange wall of a cup-shaped blank to form sharp V-shaped poly-grooves (poly-V grooves), thereby making a poly-V pulley.
According to such poly-V pulley making method, the compression process is specially important since high accuracy is required for the V-grooves formed in the flange wall of the cup-shaped blank. It is therefore not too much to say that the compression process determines the quality of a poly-V pulley manufactured. In other words, it is very difficult from a technical point of view to form corrugations in the flange wall of a cup-shaped blank, which is folded in a zigzag manner to form a grooved wall having an even or regular pitch between groove valleys in the compression process, prior to the step of finishing in which sharp V-shaped grooves are formed in the flange wall. That is, in a pre-forming process prior to compression process, a cup-shaped blank is pre-molded to have a gently corrugated flange wall provided with a series of round valleys and crests, it seldom happens that crests and valleys of the corrugation are regularly bent at the compression process in which the cup-shaped blank is compressed while rotating upper and lower rotary pressing forms with an auxiliary roller pressed into each of the valleys. Accordingly, they are unevenly or irregularly bent in most cases.
If the flange wall is bent out of shape in such manner, the pitch between valleys formed on the groove wall becomes uneven or irregular, thus making the subsequent finishing process difficult. It will be easily expected that, in the extreme case, a pulley thus manufactured would be rejected as defective. In addition, if the flange wall is irregularly bent out of shape to form uneven or irregular corrugation in the inner and outer circumferential surfaces of the flange wall, the convex and concave portions of the flange wall bite the outer circumference of the inner form disposed inside the cup-shaped blank, thus making it difficult to pull off the cup-shaped blank from the inner form, which gives rise to trouble in view of manufacturing efficiency.
As apparent from the foregoing, among the poly-V pulley manufacturing processes, the compression process is regarded as a process in which trouble may arise most easily. In the art, therefore, improvements in the compression process have been strongly desired and various proposals are made, often through the process of repeated trial and error.